Closed cellular fused silica bodies

ABSTRACT

Cellular fused silica having a bimodal closed cell structure is produced by mixing finely divided silica with finely divided boron oxynitride as a cellulating agent and heating the mixture to a temperature of at least the melting point of the silica, whereby the silica melts and is cellulated by gas generated as a result of decomposition of the boron oxynitride. The cellular silica consists of a multiplicity of primary closed cells defined by a matrix consisting essentially of silica, the matrix also containing a multiplicity of secondary macroscopic closed cells which are at least an order of magnitude smaller than the primary cells. Cellular fused silica bodies according to the invention are characterized by superior mechanical strength in addition to extreme whiteness and high purity of color, as well as other desirable properties, and are particularly useful for high temperature thermal insulation. Carefully controlled and defined shapes having very smooth surfaces may be obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of copending application Ser. No.388,688, filed Aug. 15, 1973, which is in turn a continuation of Ser.No. 137,627, filed Apr. 26, 1971, both now abandoned. See also copendingU.S. Pat. application Ser. No. 487,952, filed July 12, 1974, entitled"Method for Production of Cellular Fused Silica" which describes andclaims the method of production of the closed cellular fused silicabodies of the present invention; and copending U.S. Pat. applicationSer. No. 288,831, filed Sept. 13, 1972, now abandoned which describesthe preparation of boron oxynitride.

BACKGROUND OF THE INVENTION

The present invention relates to cellular fused silica, and moreparticularly to cellular fused silica having a bimodal closed cellstructure. The invention also relates to a method of producing suchcellular fused silica and to the compound boron oxynitride, which isespecially useful as the cellulating agent employed in the method of theinvention.

Fused silica possesses a number of highly desirable properties, such asrelative chemical inertness and resistance to attack by moisture, highelectrical resistivity, and impermeability to liquids and gases. It isparticularly known for its desirable refractory qualities, including alow thermal coefficient of expansion, high temperature resistance andhigh thermal shock resistance. Accordingly, fused silica is anexceedingly useful material in many applications including, for example,chemical apparatus, thermocouple protection devices, components ofelectronic systems, furnace parts and the like.

Dense, relatively nonporous fused silica blocks and bricks have longbeen known, being useful for the construction of refractory linings andthe like, especially in open hearth steel furnaces. Open cell fusedsilica, i.e., cellular fused silica containing a multiplicity of cellswhich are interconnected, is also well-known as a thermal insulatingmaterial. By virtue of its cellular structure, it is superior to thedense silica material in respect of its lighter weight, and also inhaving a lower thermal conductivity, which renders it more effective forthermal insulation. However, open cell fused silica is generally limitedto use in dry environments, since its interconnected cells permitpenetration of liquids. Moreover, open cell fused silica is generallycharacterized by low compressive strength and modulus of rupture ascompared with the dense material, the strength decreasing withdecreasing bulk density, thus precluding the use of open cell fusedsilica in many structural applications. For example, open cell fusedsilica available from The Carborundum Company under the trademarkSilfrax having a bulk density of 0.5 g./cc. may have a compressivestrength of only about 450 psi and a modulus of rupture of only about150 psi.

Closed cell silica, i.e., cellular silica wherein most or all of thecells are noncommunicating, has heretofore been produced, overcoming thedisadvantgeous permeability of the open cell type, but such materialshave heretofore been characterized by poor mechanical strength, just asthe open cell type. For example, U.S. Pat. Nos. 2,890,126 and 2,890,127disclose an improved method of producing cellular silica with a closedcell structure, but the highest compressive strength reported therein is125 psi. Further, due to the use of carbon and silicon carbidecontaining foaming agents, the silica foam thus made is frequently quitedark in color, often approaching black. This greatly degrades thermalinsulating ability, due to increased thermal conductivity.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a novelcellular fused silica having a very bright, clean white color due to theuse of a foaming agent having no carbon or carbide content. The materialhas a bimodal closed cell structure, i.e., the material contains cellsof two distinctly different size ranges and types, and substantially allof the cells are noncommunicating, i.e., not interconnected. Thematerial consists of a multiplicity of relatively large closed cells,herein referred to as the primary cells, which are defined by a matrixwhich consists essentially of silica. However, the silica matrix itselfadditionally contains a multiplicity of relatively small but nonethelessmacroscopic closed cells, herein referred to as the secondary cells, thesecondary cells being substantially smaller than the primary cells,i.e., by at least one order of magnitude (e.g., a power of ten). Basedupon microscopic measurements, the secondary cells in the silica matrixseldom exceed about 10 microns in size, their mean size being less thanabout 10 microns and usually considerably less than 10 microns. Incontrast, the relatively large primary cells generally have a mean sizein the range from about 0.5 mm. to about 5 mm. Both the primary and thesecondary cells may vary considerably in shape from substantiallyspherical to highly irregular. In addition to having the advantage ofbeing impervious to gases and liquids at room temperature and elevatedtemperatures, the cellular fused silica of the invention ischaracterized by a significantly higher compressive strength and modulusof rupture than closed cell silica materials heretofore available.Further, due to the absence of any carbonaceous blowing agentdecomposition products, the whiteness, or brightness, of the fusedsilica is not discolored. This factor also results in lower thermalconductivity than previously achieved.

Bodies of cellular fused silica may be produced in accordance with theinvention having bulk densities as low as about 0.4 g./cc. and up toabout 1.2 g./cc. Bodies with considerably higher bulk densities may alsobe prepared, if desired, but there is seldom any advantage in suchheavier bodies. Preferably, the bodies have a bulk density within therange from about 0.4 g./cc. to about 0.8 g./cc., such bodies providing amore or less optimum balance between light weight and mechanicalstrength. In general, the mechanical strength of the bodies tends toincrease with increasing bulk density. Bodies having a bulk density ofat least about 1.2 g./cc. may be produced having a compressive strengthof at least about 4000 psi and a modulus of rupture of at least about2000 psi. Bodies having a bulk density of about 0.4 g./cc may beproduced having a compressive strength of at least about 370 psi and amodulus of rupture of at least about 370 psi, while bodies having a bulkdensity of about 0.8 g./cc. may be produced having a compressivestrength of at least about 2200 psi and a modulus of rupture of at leastabout 1300 psi.

The method of the invention is particularly advantageous in beingsimple, short and relatively inexpensive, and in permitting thereproducible production of bodies of any desired bulk density within therange mentioned. The method is based in part upon the use of a novelcellulating agent, boron oxynitride. In accordance with the method, asubstantially homogeneous mixture is formed of finely divided silica andfrom about 0.1% to about 10%, based upon the total weight of themixture, of finely divided boron oxynitride. The mixture is then heatedto a temperature of at least the melting point of the silica employed,whereupon the silica melts. While the cellulating mechanism is not fullyunderstood, the boron oxynitride decomposes to produce gas, presumablyoxygen, which is entrapped within the molten silica and which formsclosed cells therein, producing a cellulated mass which, upon cooling,forms a rigid body of closed cell fused silica. The boron oxynitrideapparently loses oxygen during the heating and is converted in whole orin part to boron nitride, which remains in the product but which isunobjectionable in view of its excellent high temperature properties andlack of discoloration effect. When the boron oxynitride is employed inthe mixture in amounts of less than about 0.1% or more than about 10%,no appreciable cellulation occurs. Maximum cellulation is observed whenthe boron oxynitride is employed in the preferred amount of from about0.3% to about 3.0%. In general, the mean size of the primary cells tendsto increase with increasing amounts of boron oxynitride employed, andfor reasons mentioned hereinafter, relatively small mean sizes of theprimary cells are preferred. Accordingly, it has been found mostpreferable to employ the boron oxynitride in an amount of about 0.5%,which is within the range which produces maximum cellulation but whichis low enough to result in relatively small primary cells.

DESCRIPTION OF PREFERRED EMBODIMENTS

Boron oxynitride suitable for the practice of the present invention mayreadily be prepared by heating boric acid in an ammonia atmosphere,increasing the temperature gradually or stepwise to a final temperaturewithin the range from about 700° to about 1300°C and continuing theheating at the final temperature until the desired composition isobtained. As the temperature rises, the boric acid slowly liberateswater to produce boric oxide, which reacts slowly with the ammonia andis thereby nitrided to produce boron oxynitride. It will be apparentthat boric oxide may be used as a starting material instead of boricacid, if desired. The temperature increase should be sufficiently slowas to avoid melting the boric oxide or intermediate products formedtherefrom before the desired production of boron oxynitride occurs.Preferably, the boric acid is mixed with a suitable carrier materialsuch as tricalcium phosphate to avoid agglomeration of the boric oxide,the tricalcium phosphate subsequently being leached out of the product,e.g., with dilute aqueous HCl. The preparation of boron oxynitride isillustrated in copending appliction Ser. No. 288,831, filed Sept. 13,1972 now abandoned.

Boron oxynitride is a compound consisting of boron, oxygen, andnitrogen, but as with certain other compounds such as boron carbide, theproportions of its constituents are not rigorously governed bystoichiometry and the law of constant composition. Rather, boronoxynitride is a compound of somewhat variable composition in that theproportions of oxygen and nitrogen in boron oxynitride may vary withincertain limits, subject to the limitation that for each mole of boron,there must be precisely 1 mole of nitrogen and oxygen taken together.Thus, the compound may be represented by the formula BN₁ _(-m) O_(m).

A series of boron oxynitride compounds have been prepared and tested fortheir utility as cellulating agents. This series of compoundscorresponds to boron oxynitride having the formula BN₁ ₋ m O_(m) whereinm is a number from about 0.05 to about 0.3 and they have been found tobe particularly suitable as cellulating agents. Accordingly, it ispreferred that the boron oxynitride employed in the method of thepresent invention contain at least about 3% oxygen and have a maximumoxygen content of about 18%, this range of oxygen content correspondingto the stated range for the values of m. It has been observed thatmaximum cellulation occurs when the boron oxynitride contains about 13%oxygen.

EXAMPLE

Finely divided boron oxynitride is intimately mixed with finely dividedsilica to obtain a substantially homogeneous mixture containing 1% boronoxynitride, based on the total weight of the mixture. The silica, of thequartzite crystal form, analyzes 99.6% SiO₂, and has a maximum particlesize of about 50 microns, a mean particle size of about 8 microns, and amelting point of about 1680°C. The mixture is employed in a series ofruns to produce cellular fused silica bodies according to the inventionhaving various bulk densities. For each run, a graphite mold is employedwith inner dimensions 22.8 × 11.4 × 6.4 cm., the inner surfaces beingcovered with a smooth coating of boron nitride, the mold being providedwith a tightly fitting graphite cover, the inner surface of which isalso covered with a smooth coating of boron nitride.

For each run, the mold is charged with the amount of mixture shown inthe first column of the following table. The closed mold is then placedin a resistance heated carbon tube furnace and heated in a current ofnitrogen to 1700°C, whereupon the mold is removed to the atmosphere andallowed to cool to room temperature, the resulting body then beingremoved from the mold.

The body produced in each run is a 22.8 × 11.4 × 6.4 cm. brick ofcellular fused silica having a bimodal closed cell structure asdescribed above, but having a very smooth, dense fused silica surfaceabout 2 mm. thick which is substantially devoid of primary cells, eachbrick being extremely white and having very sharp edge and cornerdefinition. The bodies consist essentially of silica, analyzing 99%SiO₂, and their properties are set forth in the following table.

                                      TABLE I                                     __________________________________________________________________________    Amount of                                                                           Bulk Compressive                                                                          Modulus of                                                                          Thermal Conductivity                                  Mixture                                                                             Density                                                                            Strength                                                                             Rupture                                                                             (600°C)                                        g.    g./cc.                                                                             psi    psi   cal./sec./cm..sup.2 /°C/cm.                    __________________________________________________________________________     660  0.4   370    370  0.00071                                               1000  0.6  1330    830  0.00109                                               1330  0.8  2200   1300  0.00148                                               2000  1.2  4000   2000  0.00233                                               __________________________________________________________________________

In order to obtain a substantially homogeneous mixture of the silica andboron oxynitride with these materials in intimate contact, the boronoxynitride and silica should be finely divided, and generally the finerthe better. It is preferred that the boron oxynitride have a maximumparticle size of about 300 microns or less and a mean particle size ofabout 15 microns or less. The silica preferably has a maximum particlesize of about 200 microns or less and a mean particle size of about 10microns or less, and in general, the smaller the particle size, thesmaller the primary cells.

Any of a wide variety of types of silica may be employed in the practiceof the invention including, for example, quartz, tridymite,crystobalite, amorphous silica, fused silica powder and silicic acid.Relatively impure silica may be employed, if desired, although it is tobe noted that the purity of the final product depends primarily upon thepurity of the silica employed. In general, it is preferred to userelatively high purity silica since it is comparatively inexpensive andimparts its inherent desirable properties to the final product. Variousoxidic impurities such as alumina, sodium oxide, potassium oxide,calcium oxide, ferric oxide, magnesia and titania are preferably avoidedsince they tend to increase the thermal conductivity of silica and alsotend to reduce the melting point of the bodies produced. The silica ismixed with the desired amount of the boron oxynitride by any convenienttechnique such as dry or wet blending. Dry blending is most convenientalthough wet blending may in some cases give a slightly more intimatemixture.

If desired, the mixture may simply be placed in a tray and subjected tothe heating step, or it may be compressed into a self-sustaining shapewhich is subjected to the heating step. In either case, cellulation andexpansion will occur and a cellular fused silica body is obtained.Preferably, however, the mixture is placed in a suitable mold in orderto produce a body having the desired shape. Suitable molds may be madeof mullite, sintered alumina, sintered magnesia, boron nitride,refractory metals and the like. Carbon or graphite molds may also beemployed, but since these materials tend to react extensively with thesilica in the mixture, they are preferably coated on their innersurfaces with a material such as boron nitride. By employing molds ofvarious shapes, cellular fused silica bodies of any of a wide variety ofsimple or complex shapes may be produced, such as blocks, bricks, pipesand the like. It has been found that the texture of the inner surfacesof the mold which come in contact with the bodies is accuratelytransferred to the corresponding outer surfaces of the bodies produced,and accordingly, it is preferred that the inner mold surfaces be assmooth as possible, in which case the surfaces of the resulting bodiesare comparably smooth. Moreover, these smooth surfaces consist of densefused silica having virtually no primary cells. Such smooth, densesurface layers may be up to several millimeters in thickness and areimpervious to liquids and gases.

In accordance with a particularly preferred embodiment of the invention,a closed mold is employed, i.e., a mold of the desired configurationwhich has an opening to permit insertion of the mixture but which isprovided with a cover for the opening, the assembly being mechanicallystrong enough to withstand the internal gas pressure generated duringthe heating step. By employing such closed molds, cellular silica bodiesmay be produced in various shapes and with any desired bulk densitywithin the range from about 0.4 g./cc. to about 1.2 g./cc. or more. Aslong as the mold is charged with a weight of the mixture calculated toform a body having a bulk density of at least about 0.4 g./cc. if thatweight occupied the entire volume of the mold, the mixture willcellulate and expand to completely fill the mold. If larger amounts ofthe mixture are employed, they necessarily produce bodies havingproportionately higher bulk densities, since expansion beyond theconfines of the mold is precluded. Accordingly, the bulk density of thefinal body is a function of the volume of the mold and the weight of themixture placed therein, and cellular fused silica bodies having bulkdensities from about 0.4 g./cc. to about 1.2 g./cc. or considerablyhigher may readily and reproducibly be made. The cellulated mass expandsto contact the entire inner surface of the mold and its cover, andassuming that the entire inner surface is smooth, bodies may be producedin accordance with the invention of any desired bulk density within thespecified range with their entire surface being smooth, dense andimpervious. Closed rectangular molds may thus be employed to producecellular fused silica bricks having exceptionally sharp edge and cornerdefinition without any necessity for machining.

The heating step is carried out by heating the mixture to a temperatureof at least the melting point of the silica employed. The melting pointof silica is subject to some variation depending upon the type of silicaand the nature of the impurities therein. Preferably a temperature fromabout 10° to about 50°C above the melting point of the silica isemployed in order to favor uniform melting within a relatively shorttime. There is generally no advantage to employing higher tempertures,although much higher temperatures of 2000°C and higher may be employed,if desired. It should be noted, however, that the size of the primarycells tends to increase with increasing temperatures, as a result of thelower viscosity of the molten silica and the greater volume of the gasliberated by the cellulating agent, and thus unnecessarily hightemperatures are preferably avoided. The more rapidly the mixture isheated to the desired temperature, the better, since the faster theheating rate, the smaller the primary cells tend to be. It will thus beapparent that, while any of a wide variety of furnaces may be employedwhich are capable of generating the requisite temperatures, it will bepreferred to employ furnaces which are capable of such rapid heating.Insofar as the product is concerned, the atmosphere during heating isnot critical, air, nitrogen, the inert gases and the like being equallysuitable. When carbon or graphite molds are employed, however, it isgenerally desirable to carry out the heating step in a nonoxidizingatmosphere to avoid adverse effects on the mold, the same applying tocarbon or graphite internal furnace parts.

After the desired temperature has been reached, the resulting cellulatedmass is cooled, whereupon it forms a rigid body of cellular fused slica.The cooling is preferably carried out rapidly to avoid or minimizecrystallization of the silica.

Chemically, the resulting bodies consist essentially of silica, althoughthey may also contain impurities derived from the starting material aswell as residual boron nitride from the cellulating agent. The bodiesare generally very bright white. By employing a relatively small amountof the cellulating agent and by also employing highly pure silica,bodies may be produced in accordance with the invention which contain99% or more silica. In general, the bodies of the invention have thechemical properties characteristic of silica, being stable to variousacids and corrosive gases, even at elevated temperatures.

The cellular fused silica bodies of the invention have outstandingphysical properties. They are impervious to gases and liquids, both atroom temperature and at elevated temperatures. They have very lowthermal expansion coefficients. Their thermal shock resistance isindicated by the fact that the bodies may be heated to 1700°C andabruptly immersed in water at room temperature without cracking,spalling or any other observable effect. The bodies have outstandingmechanical strength, the compressive strength and the modulus of rupturetending to increase with increasing bulk density and also tending toincrease with decreasing size of the primary cells for any given bulkdensity. The bodies are also characterized by low thermal conductivity,which tends to decrease with decreasing bulk density and which isapparently relatively unaffected by the size of the primary cells. Thevarious desirable properties render the bodies especially useful as hightemperature refractory thermal insulation in such apparatus asindustrial furnaces and kilns, coke ovens, reaction chambers and thelike.

Percentages referred to herein are by weight except as otherwiseindicated. Modulus of rupture and compressive strength is determined inaccordance with A.S.T.M. Designation C133-55. The mean size of theprimary cells is measured by the linear intercept method using aconversion factor 1.16. Bulk density is determined by weighing thespecimen in air and in water.

While the invention has been described herein with reference to certainexamples and preferred embodiments, it is to be understood that variouschanges and modifications may be made by those skilled in the artwithout departing from the concept of the invention, the scope of whichis to be determined by reference to the following claims.

I claim:
 1. A cellular fused body consisting essentially of SiO₂ and afraction of boron nitride equivalent to from about 0.3 percent to about3.0 percent by weight of boron oxynitride of the formula BN₁ _(-m) O_(m)where m is a number from about 0.05 to about 0.3, having a bimodalclosed cell matrix structure ofa. a multiplicity of primary closed cellshaving a mean size of from about 0.5 mm to about 5 mm; and b. amultiplicity of secondary closed cells having a mean size not exceeding10 microns in size; said cellular fused body having a bulk density of atleast about 0.4 g./cc. and a thermal conductivity at 600°C of notgreater than 0.00233 cal./sec./cm.² /°C/cm.
 2. A cellular fused body asset forth in claim 1 having a smooth surface layer at least 2 mm thickwhich is substantially free of said primary closed cells having a meansize of from about 0.5 mm to about 5 mm.
 3. A cellular fused body as setforth in claim 2 in the shape of a rectangular solid, 22.8 × 11.4 × 6.4cm. brick.
 4. A cellular fused body as set forth in claim 1 having abulk density within the range from about 0.4 g./cc. to about 1.2 g./cc.,a compressive strength of at least about 370 psi and a modulus ofrupture of at least about 370 psi.
 5. A cellular fused body as set forthin claim 4 having a bulk density of at least about 0.8 g./cc., acompressive strength of at least about 2200 psi and a modulus of ruptureof at least about 1300 psi.